1. Field of the Invention
The present invention relates generally to an operating system of an electron beam source, and to an operating method of the same. It particularly concerns an operating system and an operating method for an electron beam source suitable for a flat display device.
2. Description of the Prior Art
Hitherto, flat display devices are known which display numerals or characters by selectively extracting electron beams from a selected one of several electron beam sources, each source consisting of one line cathode and at least an extracting electrode. For example, such devices are available from Digitron or Itron (trademarks for vacuum fluorescent displays manufactured by Isedensi Kogyo Kabushiki Kaisha of Japan, respectively). In such known display devices, since the area of the display section is relatively small and since no halftones are required, non-uniformity of the electron beam source has not been a grave problem.
However, in the case of a flat display device having plural line cathodes, a control grid and a phosphor screen are used for displaying the picture image, such as a television picture which has a relatively large size and requires various halftones. Therefore, an electron beam source capable of providing a uniform current density all over the display area is required in order to afford uniformity of brightness all over the display area.
Furthermore, in a display system using deflection of electron beams, and which has wires connected between the electrodes and their driving circuits, in order to improve the resolution of the picture image or in order to simplify the configuration of the device by reducing number of electrodes, uniformity of the energies of the electron beams over the display area of the device is required. Non-uniformity of energy of the electron beam or non-uniformity of electron beam density of the electron beam source are classified into a horizontal non-uniformity and a vertical non-uniformity. The horizontal non-uniformity is a non-uniformity between respective positions along an axial direction of the line cathode, and may be referred to as axial non-uniformity. The latter non-uniformity is a vertical non-uniformity which is a non-uniformity between respective positions in the vertical direction. The axial non-uniformity is mainly caused by potential variations along the axial length of the line known prior art includes.
As a known art proposed by a part of members of U.S. Pat. No. 4,227,117 (certain inventors of which are also inventors of the present invention). FIG. 1 and FIG. 2 of this application show the configuration of the electron beam source in the above-mentioned United States Patent. FIG. 2 shows a cross-section of the invention of a part of the above-mentioned United States Patent. The apparatus has a line cathode 1, a back electrode 2, and an electron-extraction electrode 3. The line cathode 1 is made by coating an electron emitting oxide material on the surface of a tungsten wire of several tens of .mu.m in diameter, and a heating current is passed through the tungsten wire. The back electrode 2 is configured in U-shaped sections which surround each line cathode 1, and is usually configured in a consecutive configuration as shown in FIG. 2. The electron-extraction electrode 3 is isolated from the back electrode 2, and has a series of apertures 4 arranged in front of the line cathode 1 so as to extract electrons and emit them through the apertures to make electron beams. The equipotential lines 8 are shown in FIG. 2.
The conventional electron beam source with the line cathode is configured as shown in FIG. 3, wherein the components designated by numerals 1, 2, 3 and 4 are those described with respect to FIG. 1 and FIG. 2. In actual use in a flat type cathode ray tube, an acceleration electrode 5 having a series of apertures 51 (or a slit in place thereof) is disposed in parallel with the electron-extraction electrode 3 with a predetermined gap inbetween and in insulated relation therewith. One end of the line cathode 1 is connected through a resistor R to a positive end of a power source V1. The other end of the line cathode 1 is connected to an anode of a diode 6, and a cathode of the diode 6 is connected to a negative end of the power source V1. A negative pulse generator 7 is connected by its output terminal 71 to the above-mentioned one end of the line cathode 1, and by its other end to the common connected ground point G, i.e. the cathode of the diode 6. To the back electrode 2, a negative end of a second power source V2 is connected and the other end of the second power source V2 is connected to the common connected ground point G. To the electron-extraction electrode 3, a positive end of a third power source V3 is connected and a negative end of the third power source V3 is connected to the common connected ground point G. To the acceleration electrode 5, a positive end of a fourth power source V4 is connected and a negative end of the fourth power source V4 is connected to the common connected ground point G.
The above-mentioned conventional system operates as follows. When the output of the negative pulse generator 7 is zero volts, the line cathode 1 is heated by a current fed from the first power source V1 and the electron-extraction electrode 3 is impressed with positive potential from the power source V3. Therefore, no electron beams are emitted through the apertures 4 and 51 since the back electrode 2 (which surrounds three sides of the line cathode 1) is impressed with a negative potential by the second power source V2. That is, the back electrode 2 functions to prevent emission of electrons from the line cathode 1 upon the application of the negative potential. In such a state, when a negative pulse potential is applied from the negative pulse generator 7 to one end of the line cathode 1, the potential of the line cathode 1 becomes negative in relation to the back electrode 2, and therefore, the line cathode 1 emits electrons. At that time, the diode 6 becomes reversely biased by the output potential of the negative pulse generator 7, and therefore turns OFF because of the negative bias, hence stopping the flow of the heating current from the first power source V1 to the line cathode 1. Therefore, in this system no potential gradation is generated along the line cathode 1 by means of the heating current.
However, the system of the above-mentioned circuit connection has the shortcoming that the different positions along the line cathode have different potentials due to potential falls caused by the current of the electron emission itself. This phenomenon is described with reference to FIG. 4.
The same number of electrons as are emitted from various points of the surface of the line cathode 1 are fed from the negative pulse generator 7 and flow through the line cathode itself which has its own resistance. Accordingly, the current induced by the electron emission from the line cathode 1 flows in an opposite direction as the flow of the emission electrons, flowing from right to the left in FIG. 4. Accordingly a voltage drop is induced along the line cathode 1. That is, the potentials on various points of the line cathode 1 are not the same. Such differences in potentials along the line cathode 1 cause differences in the currents of the electron beams themselves depending on positions along the line cathode 1. Such differences produce non-uniformity of brightness of the image produced.